Conventionally there are proposed some retroreflective sheets having excellent entrance angularity, observation angularity, and rotation angularity.
For example, U.S. Pat. No. 2,310,790 to Jungersen proposes a cube corner retroreflective article having an inclined optical axis, and such retroreflective article having the inclined optical axis has an improved entrance angularity.
European Patent No. 137,736B1 to Hoopman also discloses a triangular pyramidal cube corner retroreflective article similarly having an inclined optical axis, and a direction of the inclined optical axis is opposite to the retroreflective article disclosed by Jungersen (negative inclination). Although the retroreflective performance is improved in an azimuth of the inclined optical axis and an azimuth perpendicular thereto, the retroreflective performance is not improved in other azimuths.
U.S. Pat. No. 5,138,488 to Szczech also discloses a triangular pyramidal cube corner retroreflective article similarly having an inclined optical axis. Similarly to the invention by Hoopman, although the retroreflective performance is improved in the azimuth the inclined optical axis and the azimuth perpendicular thereto, the retroreflective performance is not improved in other azimuths.
In the three inventions described above, entrance angularity is improved by utilizing an optical principle that the retroreflective performance is improved in the direction of the inclined optical axis. However, the retroreflective performance is not improved in azimuths except for the azimuth of the inclined optical axis and the azimuth perpendicular thereto.
There are proposed various techniques relating to the improvement of the observation angularity.
In U.S. Pat. No. 4,775,219 to Appeldorn, some retroreflective elements having various vertex angle deviations are disposed in a repetitive pattern to slightly diffuse a retroreflective light beam, thereby improving the observation angularity.
With a triangular pyramidal cube corner retroreflective sheet disclosed in U.S. Pat. No. 5,706,132 to Nestgarde, using a group of triangular pyramidal cube corner elements, the directions of the inclined optical axes are defined to be two directions orthogonal to each other, the same retroreflective performance can be obtained regardless of whether the sheet is placed in the longitudinal direction or the lateral direction. However, because the array of elements having inclined axes orthogonal to each other respectively forms independent zones, the sheet is significantly disfigured.
There is also known a triangular pyramidal cube corner retroreflective element (also referred to according to equilateral element) in which lengths of three base lines of the retroreflective element are different from one another.
U.S. Pat. No. 5,926,314 to Smith et al. discloses an inequilateral retroreflective element in which the lengths of the three base lines of the triangular pyramidal cube corner retroreflective element are different from one another. In the patent document, all the formed retroreflective elements have rotationally-symmetrical shapes.
Japanese Patent Publication Laid-Open No. 11-305017 to Mimura et al. discloses an inequilateral retroreflective element in which the lengths of the three base lines of the triangular pyramidal cube corner retroreflective element are different from one another, and depths of the bases lines of the inequilateral retroreflective element are different from one another. However, all the formed retroreflective elements have rotationally-symmetrical shapes.
In a triangular pyramidal cube corner element disclosed in Japanese Patent Publication Laid-Open No. 2001-264525 to Mimura et al., a shared base line (A-B) of an element pair included in a first element pair group does not pass through intersections (A1 and B1) where other two sets of base lines (B-C1 and C1-A; and B-C2 and C2-A) intersect, a base plane of one of the elements is formed into a pentagon (A-A1-C1-B1-B), a base plane of the other element is formed into a triangle (A-C2-B), and a plane (Sc plane) defined by the shared base line (A-B) of an element pair included in the first element pair group is formed with a depth which is different from that of a common plane (S plane) defined by the two base lines (B1-C1, C1-A1 and B-C2, C2-A). However, all the formed retroreflective elements have the rotationally-symmetrical shapes.
There is also proposed a technique of forming a cube corner retroreflective element using a thin plate. Such technique is disclosed in Japanese Patent Nos. 3310297 and 3356290 to Mimura et al., International Patent Application Publication No. 2000-504821 to Klink et al., International Patent Application Publication No. 2002-507945 to Irwin et al., International Patent Application Publication Nos. 2002-509495 and 2002-508085 to Smith at al., International Patent Application Publication No. 2002-507944 to Benson et al., International Patent Application Publication Nos. 2006-520019 and 2006-520711 to Smith, and International Patent Application Publication No. 2006-520712 to Smeenk et al., for example. However, all the formed retroreflective elements have the rotationally-symmetrical shapes.